Thermal transfer recording is generally carried out by heating a transfer recording material comprising a support having thereon a color forming layer containing a sublimable or vaporizable dye to sublime or vaporize the dye and transferring the dye to an image-receiving sheet to form a dye image.
More specifically, as shown in the FIGURE of the accompanying drawing, transfer recording material 1 and image-receiving sheet 2 composed of image-receiving layer 6 and support 7 are interposed between drum 8 and heating source 3, and color forming layer 5 of transfer recording material 1 is heated from a side of support 4 opposite to the side on which the color former layer 5 is present using an electrically-controlled heat source, such as a thermal head to sublime or vaporize the dye present in color forming layer 5. The sublimed or vaporized dye is thus transferred to image-receiving layer 6 to achieve thermal transfer recording.
The material forming image-receiving layer 6 is selected depending on the kind of the color former to be transferred thereto. For example, in using a heat-fusible color former, support 7 by itself can serve as an image-receiving layer. In using a sublimable disperse dye as a color former, a coated layer comprising a high molecular weight polymer, such as polyesters, may be used as an image-receiving layer.
However, conventional image-receiving recording sheet 2 has a surface roughness of from 5 to 15 .mu.m or a thickness variation of from 10 to 20 .mu.m per mm due to the thickness unevenness or surface roughness of support 7. This roughness or unevenness may be reduced by super-calendering to some extent but the extent is limited, and a roughness of from 3 to 5 .mu.m or unevenness of 10 .mu.m or more per mm still remains on the supercalendered surface. As a result, accurate dye transfer corresponding to image signals cannot be achieved even with sublimable color formers, and even less so with heat-fusible color formers. As a result, the resulting image suffers from defects, such as dot disappearance or deficiency, and roughness in intermediate tone.
Currently employed supports for the image-receiving layer include paper, opaque synthetic paper comprising a stretched film of a propylene resin containing an inorganic fine powder (see U.S. Pat. No. 4,318,950, JP-B-46-40794, the term "JP-B" as used herein means an "examined published Japanese patent application"), and coated synthetic paper comprising a transparent polyethylene terephthalate film or a transparent polyolefin film having coated thereon an inorganic compound, e.g., silica or calcium carbonate, dispersed in a binder to increase the whiteness and dyeability.
Considering after-use characteristics of an image-receiving recording sheet with a transferred dye image as to, for example, copying suitability, pencil writability, and record preservability, it is accepted that a synthetic paper comprising a void-containing stretched film of a polyolefin resin containing an inorganic fine powder is preferred as a support from the standpoint of strength, dimensional stability, and contact with a printing head, as disclosed in JP-A-60-245593, JP-A-61-112693 and JP-A-63-193836 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In this type of synthetic paper obtained by stretching a polyolefin resin film, microvoids are formed in the film by stretching the film at a temperature lower than the melting point of the polyolefin resin so as to achieve opacity, softness to the feel, intimate contact with a printing head, and smoothness in paper feeding or discharge.
The surface temperature of an image-receiving sheet on transfer recording reaches, momentarily, 190.degree. to 200.degree. C. However, the melting point of the polyolefin resin film is 167.degree. C. or less, which is considerably lower than that of polyethylene terephthalate or a polyamide (240.degree. to 255.degree. C.). As a result, the image-receiving recording sheet with a transferred dye image thereon tends to curl inward due to the heat involved in transfer recording as disclosed in JP-A-60-245593 and JP-A-61-283593.
In order to prevent curling, a polyolefin resin film may be laminated with other base resin film and the stretching temperature can be set in the vicinity of the melting point of the base resin. In this case, however, the base resin has reduced opacity and reduced softness to the feel.